On-board antenna units disposed on vehicle body roofs are broadly classified into a single case structure and a double case structure.
The single case structure has one case fulfilling both a waterproof function for an antenna element and a function as a painted exterior case. In this case, the case must be painted in accordance with a paint color of a vehicle and the number of antenna units corresponding to the number of set colors of the vehicle must be stored as stock in a factory, putting pressure on a storage space of a manufacturing line.
The double case structure is a structure having an inner case covering an antenna element and an outer case painted in a color matching a paint color of a vehicle. In the double case structure, only the outer cases need to be prepared in accordance with the number of set colors of the vehicle.
Antenna units having the double case structure is disclosed in following Patent Documents 1 and 2.